This invention relates generally to optical devices such as optical switches and dynamically programmable attenuators. In particular, the invention provides implementations for switching optical signals among optical waveguides by means of micro-mechanical displacement of structures. Application of the invention facilitates mechanical optical switches in an integrated photonic circuit. The switches of this invention may be monostable wherein the switch always assumes a specific one of its two valid states when the drive signal is removed, or may be bistable wherein the switch remains in it""s most recent valid state when the drive signal is removed. The switches of this invention may also be set within ranges of positions in the region between the valid states to provide continuously-variable optical attenuators in an integrated photonic circuit.
Fiber optics communication links are used in numerous applications and in particular are extensively utilized as the primary means of carrying telecommunications and internet traffic between, and increasingly within, concentrations of users. These optical networks and their required management become increasingly sophisticated as their reach and capabilities increases. Of particular importance is a reliable, automatable means for setting and reconfiguring the interconnections of these links at the optical level so the various optical streams may be properly routed. Although it is possible to achieve these functions by converting the optical energy to electrical signals, routing the electrical signals, and converting back to optical, much higher performance can be achieved by directly steering the optical streams. This is referred to as xe2x80x9ctransparentxe2x80x9d switching, since once the setting is configured, the optical signals follow the set routing regardless of the nature of the information they carry. The physical implementation of setting and reconfiguring these transparent interconnections is a primary domain of optical switches.
Current optical switching technology can be divided into two classifications. The first and currently dominant approach is mechanical switching, whereby the physical displacement of at least one element of the optical path within the switch changes the coupling of the optical signals at one input port from one output port to another. Mechanical switches can provide highly efficient switching, with very little of the optical energy getting out of the channel (low insertion loss) and extremely little optical energy leaking into unselected configurations (low crosstalk). Mechanical switches however tend to require rather bulky packaging to help isolate the optical paths from unwanted disturbances, and, being an assembly of mechanical movements and bulk optical devices, are not integrable with the other waveguide devices used in optical network management. Also, being an assembly with moving parts and requiring to maintain tolerances usually below 1 micron, mechanical switches elicit heightened concern for reliability issues. Recently, several approaches have been undertaken to miniaturize mechanical optical switches using the micro-mechanical structures realizable in MEMS technologies. This provides some promise for improvement of the basic mechanical reliability and the potential to place more switch elements in a single package. However, these are still just miniaturized versions of bulk switches and still have significant lengths of optical path outside of waveguides and hence require extra isolation from mechanical disturbances. Furthermore, while these approaches employ processes that can make multiple switches during a single process step, assembly constraints for interfacing fiber to the free-space switch still limit the potential for mass production and there is no accommodation for integration with other waveguide elements.
The other class of optical switches are solid-state whereby the optical paths are wholly within waveguides and the distributions of refractive index are altered by a stimulus, typically a local application of heat or electric field, to route the optical signals along selected branches of the optical path. Such switches are directly integrable with other waveguide devices, and the production methods, including the necessary fiber bonding, are better suited for mass production. Since there is no physical displacement of the alignment for the optical paths, the solid-state switches promise improved reliability over mechanical switches, particularly as evaluated over large populations of switches in real-world deployments. Also, since typical configurations of these types of switches can be continuously tuned from one switch-state to the other, they may also provide the function of a programmable optical attenuator, a function not readily realizable in mechanical switches. These types of switches as standard components represent a less mature technology than standard mechanical switches. In order to optimize the desired sensitivity to the thermal or electric-field stimulus, special materials must be used for the waveguides that invariably do not provide lossless, polarization-insensitive transmission such as is obtained in high-quality silica planar waveguides. The constructed switches may be desirably integrated into complex photonic circuits, but do not typically as yet provide as low insertion loss as mechanical switches and may not be capable of the crosstalk isolation that can be achieved in mechanical switches.
A hybrid class of optical switches is described whereby mechanical displacements are employed to provide optical switching within standard planar waveguides. The waveguides substantially define the optical paths, while micro-mechanical structures in the substrate effect small displacements of certain waveguide elements with respect to others to conduce switching of the flow of optical signals through the device. The waveguide motions are facilitated by fabricating portions of the waveguide pattern upon micro-machined platforms, including tables, beams, and ribbons, in the silicon substrate.
In one embodiment of the invention, the upper surface of such a platform in the natural or resting state (i.e. the state in which the switch is found when no force is applied to move the platform) is coplanar with the upper surface of the remaining substrate. When properly actuated, such as by electromagnetic or electrostatic fields, the platform rotates out of the plane of the substrate surface or deflects above or below that plane. This can provide several microns change in the separation of waveguide structures near the edge of the platform from waveguide structures on the corresponding edge of the substrate or another platform. There are at least two potential configurations. The first configuration has two coupled waveguides, at least one of which waveguides is movable above or below the plane of the substrate. The waveguide structures are designed such that when the platform and substrate are coplanar, light couples between the waveguide elements on the substrate and those on the platform across the edge of the platform. This coupling may be accomplished by resonant energy transfer between guides running parallel along either side of the platform-substrate separation, or simply by continuation within guides that are axially aligned across an optically small gap between the platform and the substrate. When one (or both) of the waveguides is moved vertically above or below the plane of the substrate, the waveguides no longer couple.
A second optical device utilizes a waveguide that has a cut through it, so that the waveguide runs along or parallel to the surface of the substrate and onto and off of the platform. This device can also be configured in at least two ways. In one configuration using axially-continued coupling, the waveguides are not materially continuous across the edge of the platform, and independent displacement of the waveguide on the platform can be realized with respect to the waveguide on the substrate when the platform is deflected. Any resulting gap can be kept small with respect to the wavelength of the optical signal to minimize unwanted transitional losses.
In other described configurations, no waveguide discontinuities are necessary. The platform on which a portion of the waveguide is positioned is movable so that the discontinuity between the portions of the waveguide can be closed by moving the platform. The motions required to close the axial gap may be as small as a few microns and can be actuated by external forces such as can be generated by electric and/or magnetic forces. The motions may include a small in-plane component to control the size of the physical gap at the platform edge, but the primary switching motion is the out-of-plane component.
The natural position of the moveable elements when no external force is applied is coplanar with the substrate. The optical switch or attenuator would typically be designed to be at a well-defined state for this condition and hence provide stable operation in the absence of any applied external force. Hence one may define the xe2x80x9cnormalxe2x80x9d state as being the condition where the platform surface is coplanar with the substrate. Likewise, the xe2x80x9cswitchedxe2x80x9d state may be defined to be the condition where the platform is suitably rotated or deflected out of the plane of the substrate. It is additionally possible to make the switch bistable, for instance by placing a ferromagnetic element on the bottom of the platform and a permanent magnet near the position the ferromagnetic element achieves in the switched position. In this way, when the switch deflects to the switched position, the magnet can hold the switch in that position, and the driving force may be relaxed or removed. When the switch needs to return to its normal state, an opposing impulse may be applied to push it away from the magnet and back towards the normal state. This bistable behavior is typically not available in non-mechanical types of waveguide switches.
In certain embodiments of this invention, the switching from one state to the other occurs continuously within the range of movement rather than abruptly within the displacement. For instance, in the design depicted in [0016], movements of the platform much less than the small movement required for switching will cause a continuous decrease in the portion of the optical signal in the cross state and a complementary continuous increase in the portion of the optical signal in the bar state. As such, by inducing movements within the continuum between the switched conditions it is possible to make a device that functions as a programmable optical attenuator. Since the torsion bar suspending the platform behaves as a spring, there is a monotonic increase in the driving force required for increasing angles of deflection. This permits variable attenuation to be achieved by applying a continuously-variable drive voltage to the switch actuators. In typical configurations, there may not be a linear correspondence of drive voltage to attenuation. Hence, achieving a specific level of attenuation requires a mathematical translation of drive voltage such as with an appropriate non-linear amplifier, or preferably a digital lookup table. Alternatively, a feedback circuit from an output sensor to the drive signal may be used to stabilize the attenuator to a desired level in the optical-output power. The required motions are enabled by simple fabrication of small beam-suspended platforms in the underlying substrate of the planar lightwave circuit (PLC). Such platforms would typically fall into one of three categories: tilt platforms; cantilevered beams; or suspended ribbons. In tilt-platforms, a waveguide at the edge of the platform can be moved relative to a waveguide on the corresponding edge of the substrate by simple rotation of the platform. Likewise for a cantilevered beam, flexing the beam produces the same type of motion for the waveguide elements. Suspended ribbons can be elastically flexed and waveguides upon these beams can be moved correspondingly. Each of these categories will be discussed more fully below. A significant property of all these structures is that the amount of the local displacement on the platform surface gradually vanishes toward the attachment points that suspend the platform from the substrate. This allows the waveguides to be routed between the substrate and the platform crossing these attachment points with no material gap in the waveguide. When the beam is flexed, or platform rotated, the waveguide crossing the moveable part merely twists a bit.
Among other factors, this invention is based upon the technical findings that planar waveguide structures can be realized with substrates that are machined to provide platforms that can be deflected out of the plane of the substrate, and furthermore waveguide films can be patterned such that waveguides can be routed without separation between these platforms and the main body of the substrate by passing along the attachment features. Deflection of the platform can then induce relative displacements of waveguide elements routed to or along the edge of the platform with respect to other waveguide elements routed nearby to the corresponding edge of the main body of the substrate. This can be used to either suppress coupling between waveguides displaced along the boundary of a platform, or to open a discontinuity and induce reflections of optical signals in waveguides displaced across such a boundary. A properly designed deflection of the moveable structure can alternatively be used to direct a waveguide on the moveable structure to change alignment from one waveguide on the substrate to another in a different waveguide plane layered above or below the primary waveguide plane.